1. Technical Field
The present invention relates generally to a TEM-mode quarter wavelength directional coupler, and specifically to such a coupler having coupling line elements, disposed on opposing sides of a circuit board, which determine the even and odd mode impedances of the coupler independent of the parent circuit board on which the coupler is mounted and the material from which the circuit board is composed.
2. Description of the Prior Art
A directional coupler is a four port device used as a power divider or combiner for electromagnetic wave transmissions. The four ports are designated Port 1, Port 2, Port 3, and Port 4. When a signal is input to Port 1, it is coupled into Ports 2 and 3 but not into Port 4. A signal input to Port 4 is similarly coupled into Ports 2 and 4 but not into Port 1. Because there is no coupling between Ports 1 and 4, these ports are known as uncoupled or isolated ports relative to each other. Signals may also be input, or result from reflections, in Ports 2 and 3. A signal input to Port 2 is coupled to Ports 1 and 4, but not to Port 3, while a signal input to Port 3 is coupled to Ports 1 and 4, but not to Port 2. Thus, Ports 2 and 3 are isolated ports relative to one another.
A directional coupler is disclosed in U.S. Pat. No. 5,539,362 to Michael J. Culling, which is assigned to the same assignee as the application herein. The coupler comprises a coupler dielectric board, which stands erect on the upper surface of a parent circuit board. Upper and lower electrically conductive elements with square interdigital teeth are disposed on each surface of the coupler dielectric board. Lead lines are disposed on the upper surface of the parent circuit board and are connected to both ends of the upper conductive elements on each side. The lower conductive elements are connected to ground.
The odd mode impedance in the directional coupler is a function of the coupling between the two upper conductive elements. The majority of electric field associated with the odd mode impedance passes through the coupler dielectric board. The even mode impedance is a function of the coupling between the upper and lower conductive elements on the same side of the coupler dielectric board. Accordingly, a significant portion of the electric field associated with the even mode impedance passes through the air surrounding the board.
The different permittivity values of the dielectric board and air lead to differing phase velocities between the fields that result in poorer coupler directivity and narrower bandwidth. The square teeth in the Culling device are intended to compensate for this difference in phase velocity by equalizing the propagation delays of each mode. The wave associated with the even mode impedance is caused to meander around the gap created by the teeth. The increase in effective path length is sufficient to correct for the difference in effective velocity.
Practical limits exist, however, on the number and size of the teeth that may be used to delay the propagation of the faster wave. As a result, the Culling device cannot correct for the effects of dielectrics with very high coefficients of permittivity. It would be desirable to provide a system capable of equalizing the phase velocities regardless of the permittivity of the material.
In accordance with one aspect of the present invention, a directional coupler comprises a coupler circuit board which is mounted substantially perpendicular to the surface of a parent circuit board. First and second upper traces are disposed on the opposing surfaces of the coupler circuit board. The coupling between the upper electrically conductive traces determines the odd mode impedance of the coupler. First and second lower traces are also disposed on opposing surfaces and connected to ground. The upper and lower traces are arranged such that the even mode impedance is determined by the coupling between the first upper trace and the second lower trace and the coupling between the second upper trace and the first lower trace.
In accordance with another aspect of the invention, a directional coupler comprises a coupler circuit board which is mounted substantially perpendicular to the surface of a parent circuit board. First and second electrically conductive traces are disposed on the opposing surfaces of the coupler circuit board. The first upper electrically conductive trace has a straight top edge and a bottom edge with a number of teeth disposed thereon. The second upper electrically conductive trace also has a straight top edge and a bottom edge with a plurality of teeth disposed thereon and offset from the teeth on the first upper trace. The upper electrically conductive traces are electrically coupled to cause the coupler to exhibit an odd mode impedance, determined by the width of the traces. This impedance is a function of the required coupling factor and characteristic impedance of the coupler.
First and second lower electrically conductive traces are also disposed on the opposing surfaces of the coupler circuit board. Each of said lower electrically conductive traces are spaced from its respective upper trace. The first lower trace has a straight bottom edge and a top edge with a number of teeth staggered relative to the teeth on the bottom edge of the first upper trace. The teeth are laterally registered against the teeth on the second upper trace, such that the area covered by the teeth of the first lower trace partially overlaps an area representing the position of the teeth of the second upper trace projected onto the surface of the coupler circuit board containing the first lower trace. The second lower trace also has a straight bottom edge and a top edge with a number of teeth staggered relative to the teeth disposed on the bottom edge of the second upper trace. The teeth are laterally registered against teeth of the first upper trace, such that the area covered by the teeth of the second lower trace partially overlaps an area representing the position of the teeth of the first upper trace projected onto the surface of the coupler circuit board containing the second lower trace.
The first upper trace is electrically coupled to the second lower trace and the second upper trace is electrically coupled to the first lower trace. This causes the coupler to exhibit an even mode impedance, determined by the overlapping area of the teeth. The value of the impedance varies as a function of the required coupling factor and characteristic impedance of the coupler.